Non-destructive evaluation entails various technologies that permit analysis of materials without destroying the object formed of the material. For example, magnetic, ultrasonic, and/or acoustic measuring instruments are used to evaluate metals in a variety of products for latent flaws, cracks, etc., in order to anticipate, and thereby obviate, a part failure.
Civil engineers use non-destructive evaluation for bridge and building inspection. As buildings age, varying amounts of deterioration occur and safety issues such as seismic tolerance become significant. Some structures can be examined by established technologies such as ultrasonics or radiography, but these have serious limitations that make them unsuitable for many situations. One particularly difficult evaluation problem is for buildings having walls constructed of block, terra cotta, and/or other masonry materials where it is not possible to visually determine the exact construction of the wall in terms of the dimensions and orientation of the constituent materials, the presence or absence of voids within the wall, and the presence of different types of materials within the walls.
In situations described above, transmission-type devices, such as radiographic instruments, are limited because one needs access to both sides of the wall. Moreover, radiographic techniques using X-rays or isotope sources are generally slow and require the building to be unoccupied. Access to both sides of the wall, for through-transmission, may not be possible where the wall is a foundation wall, for example.
Ultrasonics are limited by the fact that the transducer must be mounted on a smooth surface.
Capacitive measurements have been used to locate studs within walls, but these have been successful only in determining position of a stud concealed in plaster or drywall.
Infrared thermography can be used to image or map density differences of constituent materials within a wall, but it is difficult to set the imaging camera and requires access to both sides of the wall.